The biomass consisting of plant wastes such as chaff, straw, and thinned wood generated in agriculture, forestry, and so on, and waste wood contain large amounts of fiberous materials such as cellulose or lignin as the constituents thereof. Further, the water content of the biomass is generally about 20% or higher, which is high compared to other fuels such as coal.
In recent years, the application of the biomass consisting of the plant wastes (hereinafter simply described as “biomass”) to heat sources such as a boiler has been considered as an alternative fuel for solid fuels typified by coal. Since CO2 derived from the biomass is fixed again by plants, the use of the biomass as fuel can be deemed CO2 free. Further, due to simplification of the treatment of wastes and reduction in the usage of coal, the emission of CO2 can be reduced.
There are two main problems in the use of the biomass as the solid fuel. One is that the water content is high. If the water content is high, there are disadvantages of decrease in calorific value, increase in transportation cost, and further, increase in possibility of spontaneous combustion due to the progression of microbial actions. Therefore, the biomass is not suitable for long-distance transportation and long-term storage.
Another problem is that the crushability is poor since the major constituent is formed of fiberous materials. In general, in crushing coal, there is used a method in which coal is subjected to compression milling with rollers or balls formed of a hard material. The method of crushing coal under pressure is suitable for mass processing, and is deemed economical. However, since the biomass is soft compared to coal and contains large amounts of fiberous materials, the pulverization thereof is difficult to be achieved by the compression milling method described above, and the crushing efficiency indicating the degree of atomization with respect to the required power is low. Therefore, it has been difficult to achieve mass pulverization similarly to coal. Further, in the case of combusting the biomass with coal, the amount of the biomass allowed to be put into the crusher for crushing coal stays in a level of several percents.
As one of methods of overcoming the problems described above, there is a so-called semi-carbonization method, namely a method of thermally decomposing the biomass in an atmosphere with the temperature of about 300° C. and the oxygen of less than 10% to thereby manufacture semi-carbonized fuel. An example of the semi-carbonization method is described in Patent Document 1 and Patent Document 2.
In this method, by thermally decomposing the biomass in the atmosphere with a low oxygen concentration and the temperature of about 300° C., the moisture is removed, and the fiberous materials consisting of lignin or cellulose is decomposed. Therefore, the solid fuel after the thermal decomposition is decreased in water content and is thus increased in calorific value per unit mass. Since the fiberous materials are decomposed into the component mainly consisting of carbon, the compression milling is easy. Further, due to the thermal decomposition at low temperature of around 300° C., volatile components partially remain in the solid fuel, and thus, the ignition property is equivalent to coal.